Acidum Salicylicum, B.P. Salicylic Acid.

Salicylic acid, C6H4OHCOOH, may be obtained from the oils of wintergreen and sweet birch, or by the action of carbon dioxide on sodium phenate, the commercial varieties being distinguished as "natural" and "artificial," according as they are produced from the natural salicylates or prepared synthetically. It is also official in the U.S.P. It occurs in white prisms, or as white, light, silky crystals, the natural acid being usually less white than the artificial, and possessing a slight odour indicative of its origin. Melting-point, 156° to 157°. The natural acid is free from impurities which formerly were present in the synthetic product, and is still sometimes preferred for internal use. The artificial acid is sold in three varieties, (a) so-called physiologically pure, (b) ordinary crystals, and (c) powder; all three varieties usually comply with the official tests, but the powder is less pure than the crystals and has a slightly lower melting-point.

Soluble in water (1 in 550), boiling water (1 in 9), alcohol (1 in 3.5), ether (1 in 2), glycerin (1 in 200). It forms salicylates when dissolved in solutions of ammonium citrate, ammonium acetate, sodium phosphate, borax, alkali hydroxides and carbonates.

Action and Uses.—Salicylic acid is a powerful antiseptic; it exerts a strong inhibitory influence upon the growth of micro-organisms, and retards the action of unorganised ferments as well as the ferments responsible for alcoholic and acetic fermentation; applied to wounds it is less irritating than phenol, but strong solutions exert a destructive action upon the horny layer of the epidermis, which is softened and may easily be removed. Swallowed in powder or tablets it may cause irritation and corrosion of the mucous membrane of the mouth, throat, and stomach. Salicylic acid is used externally as an antiseptic and antipruritic in the treatment of wounds and parasitic skin diseases; it is also employed as a mouth-wash, and as a local application to diminish sweating, especially when offensive. In concentrated solutions it is employed to remove such epidermal thickenings as corns and warts, and to destroy lupus. For internal use it has been replaced, almost entirely, by sodium salicylate and acetyl-salicylic acid, which resemble it in therapeutic action. A solution of 1 in 1000 is sufficiently strong to preserve alkaloidal and similar solutions. It is employed (½ to 6 in 1000) as a preservative of food and beverages. Salicylic acid is incompatible with iron salts and spirit of nitrous ether.

Salicylic acid, 3; boric acid, 10; French chalk, 87. Used as a dusting powder for the feet when there is tenderness or hyperidrosis. Pulvis Salicylicus cum Talco of the German Pharmacopoeia contains wheat starch in place of boric acid.

Salicylic acid, in powder, 2; paraffin ointment, white, 98. Salicylic acid ointment is used as an application to the skin in chronic eczema, acne, etc.; it is also used, applied thickly on lint, as a dressing for ulcerated wounds.